The present invention relates to a diagnosis system for internal combustion engines, and more particularly to a system for diagnosing failure of a coolant temperature sensor of an engine mounted on a motor vehicle.
An electronic fuel-injection system of the type which is provided with a computer operable to compute various factors, such as throttle position, volume of intake airflow, intake manifold vacuum and coolant temperature in order to produce output signals for driving solenoid-operated injection valves is known. A coolant temperature sensor is provided to produce an output signal when the temperature rises to a predetermined value. The computer uses the output signal as an input signal representing engine warming-up. However, if a particular fault occurs, such as a disconnection of wires for the sensor or a short circuiting of the sensor, the sensor produces an output having an extreme value, which results in a failure in controlling the air-fuel ratio of injected fuel and in stalling of the engine.